That's the thing. Donkey Mong does have a much lower spawn chance. They also do spawn in completely different places too. Checknout youtube. You can spawn camp him by reloading it to the title screen then back to the arid nexus thing.

Yeah, he's the one that throws the barrels, right? I had to do the challenge "Tie Clip" to cut off his tie before getting hit by the barrels. As soon as the challenge unlocked, I proceeded to kill him. No achievement.

I even reloaded my save and found him again, and for the second time no achievement.

I was playing in splitscreen if that's the problem? I mean even if the person who I was playing splitscreen with did kill him, surely they would have received the achievement, but they didn't.

I've heard only the person who wants the achievement has to damage it.. no help at all. I'm sure thats what your problem was :P

Not sure as we killed it, but none of us got the Achievement. I then tried it on Single player and I got it. The next day I went back onto split screen and I killed Donkey Mong, but my friend got the achievement. Only reason I know I killed it, is because he was somewhere on the other side of the map and didn't put a bullet into him, and told me to kill him. So I did and the achievement popped for him, luckily.

He never showed up for me until the 2nd playthrough, and even then i was trying to farm for him, had all other missions done except the story up to that pt, i killed king mong like 10 times...i was just driving around killing stuff....